X-ray reflectometer

ABSTRACT

Reflectometry apparatus includes a radiation source, adapted to irradiate a sample with radiation over a range of angles relative to a surface of the sample, and a detector assembly, positioned to receive the radiation reflected from the sample over the range of angles and to generate a signal responsive thereto. A shutter is adjustably positionable to intercept the radiation, the shutter having a blocking position, in which it blocks the radiation in a lower portion of the range of angles, thereby allowing the reflected radiation to reach the array substantially only in a higher portion of the range, and a clear position, in which the radiation in the lower portion of the range reaches the array substantially without blockage.

FIELD OF THE INVENTION

[0001] The present invention relates generally to analyticalinstruments, and specifically to instruments and methods for thin filmanalysis using X-rays.

BACKGROUND OF THE INVENTION

[0002] X-ray reflectometry (XRR) is a well-known technique for measuringthe thickness, density and surface quality of thin film layers depositedon a substrate. Conventional X-ray reflectometers are sold by a numberof companies, among them Technos (Osaka, Japan), Siemens (Munich,Germany) and Bede Scientific Instrument (Durham, UK). Suchreflectometers typically operate by irradiating a sample with a beam ofX-rays at grazing incidence, i.e., at a small angle relative to thesurface of the sample, near the total external reflection angle of thesample material. Measurement of X-ray intensity reflected from thesample as a function of angle gives a pattern of interference fringes,which is analyzed to determine the properties of the film layersresponsible for creating the fringe pattern. The X-ray intensitymeasurements are commonly made using a position-sensitive detector, suchas a proportional counter or an array detector, typically a photodiodearray or charge-coupled device (CCD).

[0003] A method for analyzing the X-ray data to determine film thicknessis described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al.,whose disclosure is incorporated herein by reference. After measuringX-ray reflectance as a function of angle, an average reflectance curveis fitted to the fringe spectrum. The average curve is based on aformula that expresses attenuation, background and surface roughness ofthe film. The fitted average reflectance curve is then used inextracting the oscillatory component of the fringe spectrum. Thiscomponent is Fourier transformed to find the film thickness.

[0004] U.S. Pat. No. 5,619,548, to Koppel, whose disclosure isincorporated herein by reference, describes an X-ray thickness gaugebased on reflectometric measurement. A curved, reflective X-raymonochromator is used to focus X-rays onto the surface of a sample. Aposition-sensitive detector, such as a photodiode detector array, sensesthe X-rays reflected from the surface and produces an intensity signalas a function of reflection angle. The angle-dependent signal isanalyzed to determine properties of the structure of a thin film layeron the sample, including thickness, density and surface roughness.

[0005] U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure isincorporated herein by reference, also describes an X-ray spectrometerbased on a curved crystal monochromator. The monochromator has the shapeof a tapered logarithmic spiral, which is described as achieving a finerfocal spot on a sample surface than prior art monochromators. X-raysreflected or diffracted from the sample surface are received by aposition-sensitive detector.

[0006] Various types of position-sensitive X-ray detectors are known inthe art of reflectometry. Solid-state arrays typically comprise multipledetector elements, which are read out by a CCD or other scanningmechanism.

[0007] Typically, each element accumulates photoelectric charge over aperiod of time before being read out and therefore cannot resolve theenergy or number of incident X-ray photons. XRR systems known in the artthat are based on such arrays simply record the total integratedradiation flux that is incident on each element. The signals at lowangles, below the total external reflection angle, are usually muchstronger than the signals above this angle. A ratio of 10⁵ to 10⁷ inphoton flux between 0° and 3° reflections is typical. The dynamic rangeof array detection systems known in the art is substantially smallerthan this ratio. Consequently, high-order fringes at higher incidenceangles cannot generally be detected. Photon counting sensitivity isneeded in order to measure the weak signals at these angles.

[0008] A further drawback of X-ray thin film measurement systems knownin the art is their lack of spatial resolution. X-ray optics, such asthe above-mentioned curved monochromators, are capable of focusing anX-ray beam to a spot diameter below 100 μm. When -the beam is incidenton a surface at a low angle, below 1°, for example, the spot on thesurface is elongated by more than 50 times this diameter. A measurementthat is made under these circumstances provides only an average ofsurface properties over the entire elongated area. For manyapplications, such as evaluating thin film microstructures on integratedcircuit wafers, better spatial resolution is required.

[0009] Although the present patent application is concerned mainly withsystems in which a sample is irradiated by a monochromatic beam, othermethods for X-ray reflectometry are also known in the art. One suchmethod is described, for example, in an article by Chihab et al.,entitled “New Apparatus for Grazing X-ray Reflectometry in theAngle-Resolved Dispersive Mode,” in Journal of Applied Crystallography22 (1989), p. 460, which is incorporated herein by reference. A narrowbeam of X-rays is directed toward the surface of a sample at grazingincidence, and a detector opposite the X-ray beam source collectsreflected X-rays. A knife edge is placed close to the sample surface inorder to cut off the primary X-ray beam, so that only reflected X-raysreach the detector. A monochromator between the sample and the detector(rather than between the source and sample, as in U.S. Pat. No.5,619,548) selects the wavelength of the reflected X-ray beam that is toreach the detector.

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to provide improvedmethods and systems for X-ray analytical measurements, and particularlyfor measurements of thin film properties.

[0011] It is a further object of some aspects of the present inventionto provide systems for X-ray reflectometry with enhanced dynamic range.

[0012] It is still a further object of some aspects of the presentinvention to provide systems for X-ray microanalysis with enhancedspatial resolution.

[0013] In preferred embodiments of the present invention, a system forX-ray reflectometry is used to determine properties of thin films on thesurface of a sample, typically a semiconductor wafer. The sample isirradiated by a monochromatic beam of X-rays, which is focused to asmall spot size on the surface of the sample. X-rays reflected from thesurface are incident on a detector array, preferably a CCD array, eachdetector element in the array corresponding to a different angle ofreflection from the surface. Charge stored by the detector elements isclocked out of the array to a processor, which analyzes the charges toderive a fringe pattern, corresponding to the intensity of X-rayreflection from the surface as a function of angle. The X-ray optics andprocessing circuitry in the system are arranged to achieve a highdynamic range, whereby high-order fringes are plainly apparent in thereflected signal. The processor analyzes the fringe pattern based on aphysical model of thin film properties, including density, thickness andsurface roughness. The high dynamic range enables the system todetermine these properties accurately not only for the upper thin filmlayer, but also for one or more underlying layers on the surface of thesample.

[0014] As noted in the Background of the Invention, one of the majorproblems in X-ray reflectometry systems is that surface reflectivity ismuch greater at low angles (near 0°) than at higher angles. The strongflux of low-angle X-rays that is incident on the detector array tends toincrease the overall background level, making it difficult to detect theweaker high-angle flux. To address this problem, in some preferredembodiments of the present invention, the system comprises a dynamicshutter, having low-angle and high-angle detection positions. In thelow-angle position, the shutter allows the entire beam of X-rays fromthe X-ray source to impinge on the sample. In the high-angle position,however, the shutter blocks low-angle X-rays. As a result, the intenselow-angle reflections are eliminated, so that the background level atthe detector is reduced, and useful signals can be derived at highangles. The processor “stitches together” the signals received by thedetector in the low- and high-angle positions of the shutter in order toderive a single fringe spectrum with enhanced dynamic range.

[0015] Preferably, the housing and readout circuits associated with thedetector array are also designed to reduce the background level in thehigh-angle elements. Most preferably, the CCD array is coupled to thereadout circuits so that the signal output from the array to thecircuits is adjacent to the end of the array that receives thehigh-angle reflections. In this configuration, the high-angle arrayelements are read out first, giving accurate readings of the weaksignals received by these elements before the readings are contaminatedby background charge transferred from the low-angle elements.Additionally or alternatively, the detector array is protected by anevacuated enclosure, closed off by an X-ray transparent window spaced asubstantial distance in front of the array, between the array and thesample. As a result, scattering of low-angle X-rays from the window andfrom the air in the space immediately in front of the array issubstantially reduced, thus further reducing the background level at thehigh-angle elements.

[0016] As a further means for reducing both the size of the incidentX-ray spot on the surface of the sample and the excessive signal at lowincidence angles, in some preferred embodiments of the presentinvention, a dynamic knife edge is positioned over the surface.Preferably, the dynamic knife edge is operated in conjunction with theabove-mentioned dynamic shutter. For measurements at low incidenceangles, the knife edge is lowered very near to the surface, interceptingthe incident X-ray beam and thus shortening the lateral dimension of thespot on the surface (i.e., the dimension in the direction along thesurface that is roughly parallel to the beam axis). For high-anglemeasurements, at which the dynamic shutter is used, the knife edge ispreferably raised out of the way, to allow the full intensity of theX-ray beam to be used. Such operation of the knife edge allowsmeasurements to be made with high spatial resolution, particularly-atlow angles at which the lateral dimension of the spot is most greatlyelongated, while maintaining high sensitivity even at high angles.

[0017] In some preferred embodiments of the present invention, theprocessor analyzes the detector array output so-as to determine a total,lumped X-ray flux for each detector element at high intensities(typically at low angles), while effectively counting individual X-rayphotons per element at low intensities (high angles). The inventors havefound that an X-ray photon of a known energy will generate a certainaverage charge in a detector element when the photon is incident as partof a high photon flux, and a lower average charge, which may be spreadover two adjacent detector elements, at low flux. Preferably, to analyzethe array output, the processor first determines the number of photonsincident on each detector element that encountered a high X-ray flux, bydividing the total charge accumulated in these elements by the high-fluxaverage charge. Then, over the remaining detector elements, theprocessor seeks individual elements or pairs of elements having chargelevels comparable to the low-flux charge. The processor records a singlephoton count for each such element or pair of elements. This techniqueallows a single detector array, such as a CCD array, to be usedsimultaneously for both lumped flux and photon counting measurements,thus further enhancing the dynamic range with which the system canmeasure the pattern of reflection fringes.

[0018] Although preferred embodiments of the present invention aredirectly mainly toward enhancing X-ray reflectometric measurements onthin films, and particularly on semiconductor wafers, the principles ofthe present invention can similarly be used in other applications ofX-ray reflectometry, as well as in other types of radiation-basedanalysis.

[0019] There is therefore provided, in accordance with a preferredembodiment of the present invention, reflectometry apparatus, including:

[0020] a radiation source, adapted to irradiate a sample with radiationover a range of angles relative to a surface of the sample;

[0021] a detector assembly, positioned to receive the radiationreflected from the sample over the range of angles and to generate asignal responsive thereto; and

[0022] a shutter, adjustably positionable to intercept the radiation,the shutter having a blocking position, in which it blocks the radiationin a lower portion of the range of angles, thereby allowing thereflected radiation to reach the array substantially only in a higherportion of the range, and a clear position, in which the radiation inthe lower portion of the range reaches the array substantially withoutblockage.

[0023] Preferably, the radiation includes X-ray radiation, and the,lower portion of the range includes angles below a critical angle fortotal external reflection of the radiation from the surface.

[0024] Further preferably, the reflected radiation is characterized by avariation of intensity as a function of the angle due to the thin filmlayers, and when the shutter is in the blocking position, the signalgenerated by the detector assembly responsive to the reflected radiationin the higher portion of the range of angles has a reduced backgroundlevel relative to the background level when the shutter is in the clearposition. In a preferred embodiment, the sample includes one or morethin film layers, and the variation of intensity includes an oscillatorypattern. Most preferably, the apparatus includes a processor, which iscoupled to receive the signal from the detector assembly and to analyzethe oscillatory pattern to determine one or more properties of the oneor more thin film layers.

[0025] Additionally or alternatively, the oscillatory pattern includesan initial shoulder occurring near a critical angle for total externalreflection of the radiation from an outer one of the thin film layers atthe surface of the sample, and the one or more properties includes adensity of the outer thin film layer, such that the processor is adaptedto estimate the density of the outer thin film layer responsive to theshoulder, irrespective of any other one of the properties.

[0026] Further additionally or alternatively, the detector assemblyincludes an array of detector elements, and the signal is indicative ofrespective charges accumulated by the detector elements due to photonsof the radiation that are incident on the elements, and the processor isadapted to estimate, responsive to the respective charges, a number ofthe photons that was incident on each of the elements.

[0027] Preferably, the detector assembly is adapted to receive theradiation over a first integration period while the shutter is in theclear position and over a second integration period, substantiallylonger than the first integration period, while the shutter is in theblocking position, and including a processor, which is coupled toreceive the signal from the detector assembly and to combine the signalgenerated by the detector assembly during the first integration periodwith the signal generated by the detector assembly during the secondintegration period so as to reconstruct the variation of the intensityover the entire range of angles.

[0028] Preferably, the detector assembly includes an array of detectorelements, including a first element positioned to receive the radiationreflected from the sample in the lower portion of the range of anglesand a last element positioned to receive the radiation reflected fromthe sample in the higher portion of the range of angles. Mostpreferably, the detector assembly includes a readout circuit and acharge coupled device (CCD), which has an output connected to thereadout circuit and is coupled to transfer charges generated by thedetector elements responsive to the radiation from the detector elementsto the output in sequence along the array beginning with the lastelement.

[0029] Preferably, the radiation source is adapted to irradiate a spoton the sample, and the apparatus includes a knife edge, adjustablypositionable to block a portion of the radiation while the shutter is inthe clear position, so as to reduce a dimension of the spot. Mostpreferably, the radiation includes X-ray radiation, and the rangeincludes angles in a vicinity of a critical angle for total externalreflection of the radiation from the surface, and the knife edge ispositionable so as to reduce the dimension of the spot to no more than 1mm.

[0030] There is also provided, in accordance with a preferred embodimentof the present invention, radiation sensing apparatus, including:

[0031] a detector assembly, including an array of detector elements,positioned to receive X-ray photons emitted over a range of angles andto generate a signal indicative of respective charges accumulated by thedetector elements due to the photons that are incident on the elements;and

[0032] a processor, which is coupled to receive the signal from thedetector assembly and to determine, responsive to the signal, whether ahigh flux of the photons or a low flux of the photons was incident oneach of the elements, and to estimate the number of photons incident oneach of the elements by dividing the charges accumulated by the elementson which the high flux was incident by a high-flux average charge, anddividing the charges accumulated by the elements on which the low fluxwas incident by a low-flux average charge, substantially different fromthe high-flux average charge.

[0033] Preferably, the low flux is considered to be incident on one ofthe elements when no more than a single one of the photons is incidenton the element over a period during which the charges are accumulated.Most preferably, for the elements on which the low flux was incident,the processor is adapted to divide the charges accumulated by amutually-adjacent pair of the elements by the low-flux average charge,so as to determine whether one of the photons was incident on one of theelements in the pair.

[0034] Additionally or alternatively, the detector assembly is adaptedto receive the X-ray photons reflected by a sample over the range ofangles, characterized by a variation of flux of the reflected photons asa function of angle, such that the high flux is incident on the elementsin a low-angle portion of the range, and the low flux is incident on theelements in a high-angle portion of the range.

[0035] There is additionally provided, in accordance with a preferredembodiment of the present invention, a detector assembly, including:

[0036] an array of detector elements, positioned to receive radiationand to generate a signal responsive thereto, the array including a firstelement and a last element and having a length defined by a distancebetween the first and last elements; and

[0037] an evacuable enclosure having a front side and a rear sideseparated by a distance that is at least equal to the length of thearray, wherein the array is positioned at the rear side of theenclosure, and the enclosure includes a window at a front side thereof,which is adapted to allow the radiation to pass therethrough so as toimpinge on the array.

[0038] Preferably, the front and rear sides of the enclosure areseparated by a distance of at least twice the length of the array.

[0039] Further preferably, the radiation is emitted from a sampleoutside the enclosure, wherein the radiation includes X-rays reflectedfrom the sample over a range of angles, such that the first elementreceives the radiation reflected from the sample in a lower portion ofthe range of angles and the last element receives the radiationreflected from the sample in a higher portion of the range of angles.Most preferably, the detector assembly includes a readout circuit and acharge coupled device (CCD), which has an output connected to thereadout circuit and is coupled to transfer charges generated by thedetector elements responsive to the radiation from the detector elementsto the output in sequence along the array beginning with the lastelement.

[0040] There is further provided, in accordance with a preferredembodiment of the present invention, a method for reflectometry,including:

[0041] irradiating a sample with radiation over a range of anglesrelative to a surface of the sample;

[0042] receiving the radiation reflected from the sample over the rangeof angles so as to generate a low-range signal responsive to theradiation reflected in a lower portion of the range;

[0043] blocking a lower part of the range of angles, thereby allowingthe reflected radiation to reach the array substantially only in ahigher portion of the range;

[0044] receiving the radiation reflected from the sample over the rangeof angles while the lower portion of the range is blocked, so as togenerate a high-range signal responsive to the radiation reflected in ahigher portion of the range; and

[0045] combining the high-range and low-range signals to determine apattern of the reflected radiation over the range of angles, includingboth the lower and higher portions.

[0046] There is moreover provided, in accordance with a preferredembodiment of the present invention, a method for sensing radiation,including:

[0047] receiving X-ray photons emitted over a range of angles at anarray of detector elements, so as to generate a signal indicative ofrespective charges accumulated by the detector elements due to thephotons that are incident on the elements; and

[0048] determining, responsive to the signal, whether a high flux of thephotons or a low flux of the photons was incident on each of theelements;

[0049] estimating the number of photons incident on each of the elementson which the high flux was incident by dividing the charges accumulatedby the elements by a high-flux average charge; and

[0050] estimating the number of photons incident on each of the elementson which the low flux was incident by dividing the charges accumulatedby the elements by a low-flux average charge, substantially differentfrom the high-flux average charge.

[0051] Preferably, determining whether the high flux or the low flux wasincident includes determining that the high flux was incident on one ofthe elements when the charge accumulated by the element, not including abackground charge, is at least three times the high-flux average charge.Additionally or alternatively, determining whether the high flux or thelow flux was incident includes determining that the low flux wasincident on one of the elements when no more than a single one of thephotons was incident on the element over a period during which thecharges were accumulated.

[0052] There is furthermore provided, in accordance with a preferredembodiment of the present invention, a method for detecting radiation,including:

[0053] enclosing an array of detector elements in an enclosure, thearray including a first element and a last element defining a length ofthe array therebetween, the enclosure having a window at a front sidethereof, which is adapted to allow radiation to pass therethrough, andwhich is positioned at a distance from the array that is at least equalto the length of the array;

[0054] evacuating the enclosure containing the array; and

[0055] receiving the radiation at the array and generating a signalresponsive thereto.

[0056] There is also provided, in accordance with a preferred embodimentof the present invention, a method for reflectometry, including:

[0057] irradiating a sample including one or more thin film layers withradiation over a range of angles relative to a surface of the sample;

[0058] receiving the radiation reflected from the sample over the rangeof angles and generating a signal responsive to the reflected radiation,the signal having an oscillatory pattern as a function of the angle, thepattern including an initial shoulder occurring near a critical anglefor total external reflection of the radiation from an outer one of thethin film layers at the surface of the sample; and

[0059] estimating a density of the outer thin film layer responsive tothe shoulder, irrespective of any other properties of the one or morethin film layers.

[0060] Preferably, the method includes determining one or more of theother properties using the estimated density, wherein determining theone or more of the other properties includes estimating a thickness ofat least one of the layers and/or a surface roughness of at least one ofthe layers.

[0061] The present invention will be more fully understood from thefollowing detailed description of the preferred embodiments thereof,taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0062]FIG. 1 is a schematic illustration of a system for X-rayreflectometry, in accordance with a preferred embodiment of the presentinvention;

[0063]FIG. 2 is a schematic block diagram illustrating an X-raydetection assembly used in the system of FIG. 1, in accordance with apreferred embodiment of the present invention;

[0064]FIG. 3 is a schematic plot of charge amplitude collected by andetector on which X-ray photons of a given energy are incident, inaccordance with a preferred embodiment of the present invention;

[0065]FIG. 4 is a flow chart that schematically illustrates a method forprocessing signals generated by an X-ray detector array, in accordancewith a preferred embodiment of the present invention;

[0066]FIGS. 5A and 5B are schematic detail views of the system of FIG.1, illustrating the operation of a dynamic knife edge and shutter usedin the system, in accordance with a preferred embodiment of the presentinvention;

[0067]FIG. 6 is a schematic plot of X-ray reflectance signals as afunction of reflection angle, under two different sets of detectionconditions, in accordance with a preferred embodiment of the presentinvention;

[0068]FIG. 7 is a schematic plot illustrating scaling of the signals ofFIG. 6, in accordance with a preferred embodiment of the presentinvention;

[0069]FIG. 8 is a schematic plot illustrating an X-ray reflectancespectrum, formed by combining the signals of FIG. 7, in accordance witha preferred embodiment of the present invention; and

[0070]FIG. 9 is a flow chart that schematically illustrates a method forextracting thin film data from an X-ray reflectance spectrum, inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0071]FIG. 1 is a schematic illustration of a system 20 for X-rayreflectometry of a sample 22, in accordance with a preferred embodimentof the present invention. The sample is preferably mounted on a motionstage 24, allowing accurate adjustment of its position and orientation.An X-ray source 26, typically an X-ray tube with suitablemonochromatizing optics (not shown), irradiates a small area 28 onsample 22. A preferred X-ray tube for this purpose is the XTF5011 tube,produced by Oxford Instruments of Scotts Valley, Calif. A number ofdifferent optical configurations that may be used in source 26 aredescribed in U.S. patent application Ser. No. 09/408,894, which isassigned to the assignee of the present patent application and isincorporated herein by reference. The optics preferably comprise acurved crystal monochromator, such as the Doubly-Bent Focusing CrystalOptic, produced by XOS Inc., of Albany, N.Y. Other suitable optics aredescribed in patent application Ser. No. 09/408,894 and in theabove-mentioned U.S. Pat. Nos. 5,619,548 and 5,923,720. Further possibleoptical configurations will be apparent to those skilled in the art. Atypical X-ray energy for reflectometric measurements in system 20 isabout 8.05 keV (CuKal). Alternatively, other energies may be used, suchas 5.4 keV (CrKal). A dynamic knife edge 36 and shutter 38 arepreferably used to limit an incident beam 27 of the X-rays, as describedfurther hereinbelow.

[0072] A reflected beam 29 of X-rays from sample 22 is collected by adetector assembly 30. Preferably, assembly 30 collects reflected X-raysover a range of reflection angles between about 0° and 3°, both belowand above the critical angle of the sample for total externalreflection. Assembly 30 comprises a detector array 32, preferably a CCDarray, as described hereinbelow. Although for simplicity ofillustration, only a single row of detectors elements is shown in FIG.1, with a relatively small number of detector elements, in preferredembodiments of the present invention, array 32 generally includes agreater number of elements, arranged in either a linear or a matrix(two-dimensional) array. Assembly 30 further comprises a window 34 madeof a suitable X-ray transparent material, such as beryllium, spaced infront of the detector array, between the array and the sample.

[0073] A reflectometry processor 40 analyzes the output of assembly 30,so as to determine a distribution 42 of the flux of X-ray photonsreflected from sample 22 as a function of angle at a given energy orover a range of energies. Typically, sample 22 has one or more thinsurface layers, such as thin films, at area 28, so that distribution 42exhibits an oscillatory structure due to interference effects amongreflected X-ray waves from the interfaces between the layers. Theprocessor analyzes characteristics of the oscillatory structure in orderto determine the thickness, density and surface quality of one or moreof the surface layers, using methods of analysis described hereinbelow.

[0074] Although in the preferred embodiment shown in FIG. 1, system 20is described with reference to X-ray reflectometry, it will beappreciated that the system may similarly be used, mutatis mutandis, inother fields of X-ray analysis. Possible fields of application includeX-ray fluorescence (XRF) analysis, including particularly grazingemission XRF, as well as other XRF techniques known in the art. Grazingemission XRF is described, for example, in an article by Wiener et al.,entitled “Characterization of Titanium Nitride Layers byGrazing-Emission X-ray Fluorescence Spectrometry,” in Applied SurfaceScience 125 (1998), p. 129, which is incorporated herein by reference.Furthermore, the principles of system 20 may be implemented inposition-sensitive detection systems for other energy ranges, such asfor detection of gamma rays and other nuclear radiation.

[0075]FIG. 2 is a block diagram that schematically shows details ofdetector assembly 30, in accordance with a preferred embodiment of thepresent invention. As noted above, detector array 32 preferablycomprises a CCD array, such as the model S7032-0908N array produced byHamamatsu, of Hamamatsu City, Japan. This array comprises 536×256pixels, with an overall size of 12.6×6 mm, and is preferably operated ina line-binning mode, using special hardware supplied for this purpose byHamamatsu. Alternatively, the detector array may comprise an array ofPIN diodes with suitable readout circuits, possibly including integratedprocessing electronics, as described in U.S. patent application Ser. No.09/409,046, which is assigned to the assignee of the present patentapplication and is incorporated herein by reference. Application Ser.No. 09/409,046 also describes alternative features of the array,including various geometrical configurations of the array (both one- andtwo-dimensional) and masking that may be applied to enhance the array'sdetection properties. These features are applicable to assembly 30 ofthe present patent application, as well. In any event, it will beunderstood that these detector types are described here by way ofexample, and detectors of any suitable type, dimension and number can beused.

[0076] Detector assembly 30 comprises an evacuable enclosure 44 adjacentto detector array 32. The front side of enclosure 44, between array 32and sample 22, is closed off by window 34, and the enclosure isevacuated during operation. Preferably, the distance from array 32 towindow 34 is at least equal to the length of the array, measured from afirst detector element 46 to a last detector element 48, and is mostpreferably at least two to three times the length of the array. (Firstdetector element 46 is positioned to capture the lowest-angle reflectedphotons, around 0°, while last element 48 captures the highest-anglephotons, typically near 3°.) The inventors have found that removal ofthe air from the region immediately in front of the array, along withdistancing the window from the array, substantially reduces the numberof scattered X-ray photons that reach the array. When array 32 operatesin air, or when window 34 is positioned close to the array, scatter ofphotons reflected from sample 22 at low angles ordinarily makes asubstantial contribution to the signal background at high angles.Because the low-angle reflections are generally so intense by comparisonwith the high-angle reflections, this background significantly degradesor even masks the high-angle signal. The use of window 34 and evacuatedenclosure 44, as shown in FIG. 2, eliminates most of this scatterbackground, without the difficulty and expenses of having to evacuatethe entire system.

[0077] A further source of background in assembly 30 is residual chargein the CCD shift register of array 32. CCDs operate by transferringcharge in a “bucket brigade” mode, from one element to the next down thearray. The charge is thus transferred, one pixel at a time, to readoutcircuits 50, which are coupled to an output of the array at last element48. Although CCDs are highly efficient in transferring charge fromelement to element, there is still a small amount of residual chargeleft behind in each transfer, which is roughly proportional to theamount of charge transferred. In the configuration shown in FIG. 3,after each X-ray exposure period, last element 48 is read out first,while first element 46 is read out last, after its charge has beentransferred down the entire array. By positioning array 32 so that lastelement 48, which typically receives the weakest X-ray signal, is readout first, the background level due to residual charge in the weaksignal elements near element 48 is minimized. The background added tothe strong signals from detector elements near first element 46, due toreading these signals out last, is not significant by comparison withthe strength of the signals themselves.

[0078]FIG. 3 is a schematic plot showing the response of the elements ofdetector array 32 to incident photons, in accordance with a preferredembodiment of the present invention. In this experiment, the detectorarray was irradiated with monochromatic X-ray radiation, and signalswere gathered from the array in two different read-out modes. Theamplitude units of the horizontal axis are arbitrary, but indicate theestimated number of electrons generated for each X-ray photon that isincident on an element of the array, based on the output signal from thearray. Each incident photon constitutes an “event,” and the verticalaxis shows, for each value of the amplitude, how many times a charge ofthat amplitude was generated by an incident photon.

[0079] As can be seen in the figure, the distribution of events issubstantially different for the different read-out modes. The chargegenerated due to an incident photon may typically be distributed betweentwo adjacent pixels. The curve having a sharp peak near amplitude 100 isaccordingly generated by combining the charge counted in adjacentpixels. This read-out mode, however, can be applied only when the fluxis low enough so that during any given read-out frame, there isgenerally no more than one incident photon per pixel, with most pixelsreceiving no photons. At high flux, with many photons incident on eachelement, this “charge combining” approach cannot be used. In this case,the event distribution has the form of the second curve shown in FIG. 3,with a peak near amplitude 60. Such behavior was observed for both Cu Ka(8.05 keV) and Cr Kα (5.41 keV) irradiation. The inventors haveempirically found that the combination of these two read-out modes forhigh- and low-flux conditions can be used effectively in converting theelectrical signal levels received from array 32 into units of photoncounts over a very large dynamic range, as is commonly encountered inXRR measurements.

[0080]FIG. 4 is a flow chart that schematically illustrates a method foranalyzing signals received by detector array 32, in accordance with apreferred embodiment of the present invention. The method is based onthe principles described above with reference to FIG. 3. It is actuatedby processor each time charge is read out-of the elements of the array,with respect to each of the elements in turn, in order to translate thecharge amplitude measured in each element into units of incident X-rayphotons.

[0081] The method includes four operations, which are performed over allof the pixels in the array:

[0082] 1. In a background subtraction step 52, a general backgroundlevel is subtracted from the digitized signal level measured in each ofthe pixels, thus generating a background-subtracted level y(j) for eachpixel. The general background level is found by measuring the darkcurrent output of the detector array in the absence of incidentradiation.

[0083] 2. In a strong flux counting step 54, the signal levels afterbackground subtraction are compared to a high signal threshold, which isdetermined based on the high-flux mode of the distribution shown in FIG.3. Specifically, the processor finds pixels (i.e., detector arrayelements) whose signal levels, y(j), are greater than three times asignal amplitude parameter, Av1, which is determined empirically basedon the location of the peak in the high-flux single-photon signal (suchas the high-amplitude peak shown in FIG. 3). For each such pixel j, thenumber of photon counts n(j) for the pixel is determined to ben(j)=int{[y(j)−BL1]/Av1}, wherein BL1 is an empirical backgroundthreshold. Processor 40 then sets the signal level for the pixel tozero, so that it is not counted again in subsequent steps.

[0084] 3. In a pixel pair counting step 56, the processor searches theremaining pixels, not counted in step 54, to find pairs of adjacentpixels whose total signal level (summed over the pair) is roughly equalto a low-flux single-photon signal amplitude parameter Av2. Thisparameter, as well as a second background threshold BL2, is determinedempirically. Specifically, the inventors have found that setting Av2=Av1gives good results. BL1 and BL2 are set so that the high- andlow-intensity portions of the photon energy spectrum, found at steps 54and 56 respectively, will match up. Based on these parameters, at step56, the processor finds pairs of pixels that satisfy|y(j)+y(j+1)−Av2|<BL2. For each such pair, the processor records asingle photon count, which is arbitrarily assigned to the photon countn(j) for the first of the two pixels. The signal levels in these pixelsare then zeroed, as well.

[0085] 4. In a remainder counting step 58, any other pixels withsignificant signal levels that were not counted at step 54 or 56 areevaluated. To carry out this step, any pixel signal values y(j) that arebelow the background level, so that y(j)<0 after background subtractionat step 52, are set to y(j)=0. Then, for each pixel that is a localmaximum (i.e., greater signal value y(j) than its immediate neighbors),the pixel photon count n(j) receives the valuen(j)=int{[y(j)+y(k)−BL1]/Av1}, wherein y(k) is the higher of the signallevels of the two pixels, j+1 and j−1, neighboring on pixel j. Thevalues y(j) and y(k) are then set to zero. For any remaining pixels (notlocal maximum or their higher neighbors), the photon count receives thevalue n(j)=int{[y(j)−BL1]/Av1}, and y(j) is zeroed.

[0086] After all four of the steps listed above are completed, the arrayis reset, and the processor is ready to receive the next signal readoutfrom array 32. The number of counts determined for each of the pixels isaccumulated in a respective register, in a count accumulation step 60.The steps of signal readout and processing, as described above, arepreferably repeated enough times to determine a count spectrum over theentire array. Using this technique, fringe structure can be seen notonly at the low-angle, high-flux pixels, where many pixels are incidentin each signal readout cycle, but also at high angles, where only onecount or less may arrive at each pixel in a given cycle.

[0087]FIGS. 5A and 5B are detail views of system 20, illustrating theuse of knife edge 36 and shutter 38, in accordance with a preferredembodiment of the present invention. In FIG. 5A, the knife edge andshutter are positioned to allow optimal detection of low-anglereflections, near 0°. Shutter 38 is withdrawn outside the extent ofincident beam 27. Knife edge 36 is positioned to cut the upper portionof the incident beam. As a result, most of the incident beam is cut off,and the lateral dimension of the X-ray spot incident on area 28 isreduced. Preferably, the knife edge is lowered to within less than 10 μmof the surface of sample 22, and most preferably to as little as 1 μmfrom the surface. The lateral dimension of the spot is thus reduced to 1mm or less, instead of the typical dimension of 5 mm or more when theknife-edge is not used. The reduced spot size on the sample means thatlow-angle reflection measurements made by system 20 have enhancedspatial resolution, providing more detailed information about thin filmmicrostructures on sample 22. Alternatively or additionally, when acertain area of the sample, such as a patterned semiconductor wafer,must be set aside for testing, the small spot size enables a smallerportion of sample “real estate” to be used for this purpose.

[0088] Moreover, the inventors have found that wafers are prone towarping, particularly when held by a vacuum chuck, as is commonlypracticed in test and fabrication equipment. When the X-ray spot isspread over a long lateral dimension, this warping can cause differentparts of the spot to be incident on the wafer at slightly differentangles. As a result, the fringe structure in the measured distributionof the reflected radiation is blurred. Thus, an additional benefit ofthe use of knife edge 36 is the reduction of this blurring due towarping of the wafer, since the range of angles of incidence of theX-rays within the spot is accordingly narrowed.

[0089] In FIG. 5B, knife edge 36 and shutter 38 are positioned to enableeffective detection of weaker, high-angle reflection. In this case, theknife edge is withdrawn from the beam, while the shutter is positionedto cut off the low-angle portion of incident beam 27. Alternatively, theshutter may be positioned to cut off the low-angle portion of reflectedbeam 29. Only the high-angle reflections from sample 22 reach thedetector array, and not the strong low-angle reflections. As a result,the background level at the high-angle elements of the detector array isreduced, and X-ray photons can be collected by the array over asubstantially longer integration period without saturation. Thus, theweak, high-angle signals are detected with enhanced signal/noise ratio.

[0090]FIG. 6 is a schematic plot showing reflectometric signals gatheredby processor 40, in accordance with a preferred embodiment of thepresent invention, using dynamic shutter 38 as shown in FIGS. 5A and 5Band the signal processing method of FIG. 4. The plot shows, on alogarithmic scale, the number of counts n(j) accumulated at each pixelas a function of reflection angle. A high-intensity trace 70 isgenerated in the configuration of FIG. 5A (with or without the use ofknife edge 36), using a relatively short exposure. A second,low-intensity trace 72 is generated with shutter 38 positioned to blockthe low-angle beam, as shown in FIG. 5B, using a long exposure. Trace 70shows the low-angle fringe structure, while trace 72 shows thehigh-angle structure. Fringes in an intermediate region (around 1°) canbe seen in both traces.

[0091]FIG. 7 is a schematic plot showing traces 70 and 72 after trace 70has been scaled to match the amplitude of trace 70 in the intermediateregion.

[0092]FIG. 8 shows a composite trace 74, generated by combining thescaled and superimposed traces 70 and 72 of FIG.. 7, in accordance witha preferred embodiment of the present invention. For each pixel, thevalue in composite trace 74 is a weighted sum of the correspondingvalues in traces 70 and 72, with weighting factors that varyappropriately as a function of angle. Trace 74 shows a well-definedfringe pattern extending from near 0° out to 2.5°. The high-anglefringes are particularly important in determining properties of innerlayers at the surface of sample 22, when a multi-layer thin filmstructure is to be analyzed. The spikes seen at high angles areexperimental artifacts, which are ignored in the analysis describedbelow.

[0093]FIG. 9 is a flow chart that schematically illustrates a method foranalyzing trace 74 to determine the properties of thin films on sample22, in accordance with a preferred embodiment of the present invention.The method is based on a physical model of the reflected fringe pattern.According to this model, the angular position of an initial shoulder 78(FIG. 8) in the fringe pattern is determined mainly by the density ofthe uppermost layer on the sample. The spatial frequency or frequenciesof the fringes are indicative of the thickness of the film layers. Theintensity of the higher-order fringes relative to the low-order ones,indicated by a decay curve 76 fitted to trace 74, is determined mainlyby the roughness of the outer surface of the sample and, secondarily, ofthe interfaces between the film layers on the sample.

[0094] Based on this model, at a density fitting step 80, an initial,theoretical fringe pattern is fitted to trace 74, by adjusting thedensity in the model so that the theoretical pattern fits shoulder 78.For the purpose of this step, the film is considered to be infinitelythick, and only the part of the fringe pattern in the immediate vicinityof the shoulder is considered. Next, at a roughness fitting step 82, aparameter in the model corresponding to the roughness of the outersurface of the sample is adjusted so that curve 76 fits the actual,average decay of trace 74 as a function of angle. The fit is performedso that the difference between trace 74 and curve 76, integrated overthe entire angular range (or a substantial, selected portion of therange), is close to zero.

[0095] The fitted decay curve is subtracted out of trace 74, in order toisolate the oscillatory portion of the reflected signal, at anoscillation extraction step 84. The oscillation frequency or frequenciesin the subtracted signal are determined, at a frequency determinationstep 86, preferably using a Fast Fourier Transform (FFT) analysis of thesignal. The frequency spectrum is preferably filtered to eliminatespurious high-frequency components. The filtered spectrum is transformedback to the spatial domain, and a least squares fit is performed todetermine the thicknesses of the detected layers on the sample surface,at a thickness measurement step 88. Typically, when the sample has amulti-layer structure, the outer layer will give the strongest frequencycomponent in the spectrum, at a relatively low frequency correspondingto the thickness of this layer. The next frequency component will be ata higher frequency, corresponding to the combined thickness of the outerlayer and the next layer below it. The thickness of the next layer isdetermined by subtracting the outer layer thickness from the combinedthickness. Additional layer thicknesses may be determined in like mannerif the spectrum is sufficiently well resolved.

[0096] Upon completion of step 88, the physical properties of the outerlayer on the sample—density, thickness, and outer surface roughness—areall known. Assuming that more than a single frequency was found at step86, corresponding to a multi-layer structure, trace 74 can be furtheranalyzed to determine the properties of one or more inner layers. At aninner density fitting step 90, the density of the second layer (belowthe outer layer) is introduced into the theoretical model and isadjusted to produce an optimal fit. A roughness parameter for thesurface between the outermost and second layers is adjusted to improvethe fit of the model curve to the amplitude of the oscillations in trace74, at an inner roughness fitting step 92. Thickness parameters,corresponding to possible errors in the thickness of the outermost andsecond layers, are adjusted at a fine tuning step 94, in order tocorrect any mismatch between the positions of the fringes and thederived model. To the extent that trace 74 provides sufficientresolution of fine fringe detail, as noted above, steps 90, 92 and 94may be repeated for further, underlying layers on the sample.

[0097] Although the features of system 20 have been described here incombination, it will be appreciated that individual ones or subgroups ofthese features can also be used independently of the other features.Furthermore, although these features are described in the context ofX-ray reflectometry, at least some of them are also applicable in otherfields of analysis, such as diffractometry, using X-rays and otherradiation bands.

[0098] It will thus be appreciated that the preferred embodimentsdescribed above are cited by way of example, and that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and subcombinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofwhich would occur to persons skilled in the art upon reading theforegoing description and which are not disclosed in the prior art.

1. Reflectometry apparatus, comprising: a radiation source, adapted toirradiate a sample with radiation over a range of angles relative to asurface of the sample; a detector assembly, positioned to receive theradiation reflected from the sample over the range of angles and togenerate a signal responsive thereto; and a shutter, adjustablypositionable to intercept the radiation, the shutter having a blockingposition, in which it blocks the radiation in a lower portion of therange of angles, thereby allowing the reflected radiation to reach thearray substantially only in a higher portion of the range, and a clearposition, in which the radiation in the lower portion of the rangereaches the array substantially without blockage.
 2. Apparatus accordingto claim 1, wherein the radiation comprises X-ray radiation, and whereinthe lower portion of the range comprises angles below a critical anglefor total external reflection of the radiation from the surface. 3.Apparatus according to claim 1, wherein the reflected radiation ischaracterized by a variation of intensity as a function of the angle dueto the thin film layers, and wherein when the shutter is in the blockingposition, the signal generated by the detector assembly responsive tothe reflected radiation in the higher portion of the range of angles hasa reduced background level relative to the background level when theshutter is in the clear position.
 4. Apparatus according to claim 3,wherein the sample comprises one or more thin film layers, and whereinthe variation of intensity comprises an oscillatory pattern. 5.Apparatus according to claim 4, and comprising a processor, which iscoupled to receive the signal from the detector assembly and to analyzethe oscillatory pattern to determine one or more properties of the oneor more thin film layers.
 6. Apparatus according to claim 5, wherein theoscillatory pattern comprises an initial shoulder occurring near acritical angle for total external reflection of the radiation from anouter one of the thin film layers at the surface of the sample, andwherein the one or more properties comprises a density of the outer thinfilm layer, such that the processor is adapted to estimate the densityof the outer thin film layer responsive to the shoulder, irrespective ofany other one of the properties.
 7. Apparatus according to claim 5,wherein the detector assembly comprises an array of detector elements,and wherein the signal is indicative of respective charges accumulatedby the detector elements due to photons of the radiation that areincident on the elements, and wherein the processor is adapted toestimate, responsive to the respective charges, a number of the photonsthat was incident on each of the elements.
 8. Apparatus according toclaim 7, wherein the processor is adapted to determine, responsive tothe signal, whether a high flux of the photons or a low flux of thephotons was incident on each of the elements, and to estimate the numberof photons incident on each of the elements by dividing the chargesaccumulated by the elements on which the high flux was incident by ahigh-flux average charge, and dividing the charges accumulated by theelements on which the low flux was incident by a low-flux averagecharge, substantially different from the high-flux average charge. 9.Apparatus according to claim 3, wherein the detector assembly is adaptedto receive the radiation over a first integration period while theshutter is in the clear position and over a second integration period,substantially longer than the first integration period, while theshutter is in the blocking position, and comprising a processor, whichis coupled to receive the signal from the detector assembly and tocombine the signal generated by the detector assembly during the firstintegration period with the signal generated by the detector assemblyduring the second integration period so as to reconstruct the variationof the intensity over the entire range of angles.
 10. Apparatusaccording to claim 1, wherein the detector assembly comprises an arrayof detector elements, including a first element positioned to receivethe radiation reflected from the sample in the lower portion of therange of angles and a last element positioned to receive the radiationreflected from the sample in the higher portion of the range of angles.11. Apparatus according to claim 10, wherein the detector assemblycomprises a readout circuit and a charge coupled device (CCD), which hasan output connected to the readout circuit and is coupled to transfercharges generated by the detector elements responsive to the radiationfrom the detector elements to the output in sequence along the arraybeginning with the last element.
 12. Apparatus according to claim 10,wherein the array has a length from the first element to the lastelement, and wherein the detector assembly comprises an evacuableenclosure having a front side and a rear side separated by a distancethat is at least equal to the length of the array, wherein the array ispositioned at the rear side of the enclosure, and the enclosurecomprises a window at a front side thereof, which is adapted to allowthe reflected radiation to pass therethrough.
 13. Apparatus according toclaim 1, wherein the radiation source is adapted to irradiate a spot onthe sample, and comprising a knife edge, adjustably positionable toblock a portion of the radiation while the shutter is in the clearposition, so as to reduce a dimension of the spot.
 14. Apparatusaccording to claim 13, wherein the radiation comprises X-ray radiation,and wherein the range comprises angles in a vicinity of a critical anglefor total external reflection of the radiation from the surface, andwherein the knife edge is positionable so as to reduce the dimension ofthe spot to no more than 1 mm.
 15. Radiation sensing apparatus,comprising: a detector assembly, comprising an array of detectorelements, positioned to receive X-ray photons emitted over a range ofangles and to generate a signal indicative of respective chargesaccumulated by the detector elements due to the photons that areincident on the elements; and a process, which is coupled to receive thesignal from the detector assembly and to determine, responsive to thesignal, whether a high flux of the photons or a low flux of the photonswas incident on each of the elements, and to estimate the number ofphotons incident on each of the elements by dividing the chargesaccumulated by the elements on which the high flux was incident by ahigh-flux average charge, and dividing the charges accumulated by theelements on which the low flux was incident by a low-flux averagecharge, substantially different from the high-flux average charge. 16.Apparatus according to claim 15, wherein the low flux is considered tobe incident on one of the elements when no more than a single one of thephotons is incident on the element over a period during which thecharges are accumulated.
 17. Apparatus according to claim 16, whereinfor the elements on which the low flux was incident, the processor isadapted to divide the charges accumulated by a mutually-adjacent pair ofthe elements by the low-flux average charge, so as to determine whetherone of the photons was incident on one of the elements in the pair. 18.Apparatus according to claim 15, wherein the detector assembly isadapted to receive the X-ray photons reflected by a sample over therange of angles, characterized by a variation of flux of the reflectedphotons as a function of angle, such that the high, flux is incident onthe elements in a low-angle portion of the range, and the low flux isincident on the elements in a high-angle portion of the range.
 19. Adetector assembly, comprising: an array of detector elements, positionedto receive radiation and to generate a signal responsive thereto, thearray including a first element and a last element and having a lengthdefined by a distance between the first and last elements; and anevacuable enclosure having a front side and a rear side separated by adistance that is at least equal to the length of the array, wherein thearray is positioned at the rear side of the enclosure, and the enclosurecomprises a window at a front side thereof, which is adapted to allowthe radiation to pass therethrough so as to impinge on the array.
 20. Anassembly according to claim 19, wherein the front and rear sides of theenclosure are separated by a distance of at least twice the length ofthe array.
 21. An assembly according to claim 19, wherein the radiationis emitted from a sample outside the enclosure.
 22. An assemblyaccording to claim 21, wherein the radiation comprises X-rays reflectedfrom the sample over a range of angles, such that the first elementreceives the radiation reflected from the sample in a lower portion ofthe range of angles and the last element receives the radiationreflected from the sample in a higher portion of the range of angles.23. An assembly according to claim 22, wherein the detector assemblycomprises a readout circuit and a charge coupled device (CCD), which hasan output connected to the readout circuit and is coupled to transfercharges generated by the detector elements responsive to the radiationfrom the detector elements to the output in sequence along the arraybeginning with the last element.
 24. A method for reflectometry,comprising: irradiating a sample with radiation over a range of anglesrelative to a surface of the sample; receiving the radiation reflectedfrom the sample over the range of angles so as to generate a low-rangesignal responsive to the radiation reflected in a lower portion of therange; blocking a lower part of the range of angles, thereby allowingthe reflected radiation to reach the array substantially only in ahigher portion of the range; receiving the radiation reflected from thesample over the range of angles while the lower portion of the range isblocked, so as to generate a high-range signal responsive to theradiation reflected in a higher portion of the range; and combining thehigh-range and low-range signals to determine a pattern of the reflectedradiation over the range of angles, including both the lower and higherportions.
 25. A method according to claim 24, wherein the radiationcomprises X-ray radiation, and wherein the lower portion of the rangecomprises angles below a critical angle for total external reflection ofthe radiation from the surface.
 26. A method according to claim 24,wherein the reflected radiation is characterized by a variation ofintensity as a function of the angle due to the thin film layers, andwherein receiving the radiation while the lower portion of the range ofangles is blocked comprises generating the high-range signal with areduced background level relative to the background level when the lowerportion of the range is not blocked.
 27. A method according to claim 26,wherein the sample comprises one or more thin film layers, and whereinthe variation of intensity comprises an oscillatory pattern.
 28. Amethod according to claim 27, and comprising analyzing the oscillatorypattern to determine one or more properties of the one or more thin filmlayers.
 29. A method according to claim 28, wherein the oscillatorypattern comprises an initial shoulder occurring near a critical anglefor total external reflection of the radiation from an outer one of thethin film layers at the surface of the sample, and wherein determiningthe one or more properties comprises estimating a density of the outerthin film layer responsive to the shoulder, irrespective of any otherone of the properties.
 30. A method according to claim 24, wherein thedetector assembly comprises an array of detector elements, and whereinthe signal is indicative of respective charges accumulated by thedetector elements due to photons of the radiation that are incident onthe elements, and wherein combining the signals comprises estimating,responsive to the respective charges, a number of the photons that wasincident on each of the elements.
 31. A method according to claim 30,wherein estimating the number of the photons comprises: determining,responsive to the signals, whether a high flux of the photons or a lowflux of the photons was incident on each of the elements; dividing thecharges accumulated by the elements on which the high flux was incidentby a high-flux average charge; and dividing the charges accumulated bythe elements on which the low flux was incident by a low-flux averagecharge, substantially different from the high-flux average charge.
 32. Amethod according to claim 24, wherein receiving the radiation so as togenerate the low-range signal comprises receiving the radiation over afirst integration period, and wherein receiving the radiation so as togenerate the high-range signal comprises receiving the radiation over asecond integration period, substantially longer than the firstintegration period.
 33. A method according to claim 24, whereinreceiving the radiation comprises receiving the radiation at an array ofdetector elements, including a first element positioned to receive theradiation reflected from the sample in the lower portion of the range ofangles and a last element positioned to receive the radiation reflectedfrom the sample in the higher portion of the range of angles, andtransferring charges generated responsive to the radiation from thedetector elements in sequence out of the array beginning with the lastelement.
 34. A method according to claim 24, wherein receiving theradiation comprises: receiving the radiation at an array of detectorelements contained in an enclosure, the array including a first elementand a last element defining a length of the array therebetween, theenclosure having a window defining a front side thereof, which isadapted to allow the reflected radiation to pass therethrough, and whichis positioned at a distance from the array that is at least equal to thelength of the array; and evacuating the enclosure containing the arraywhile receiving the radiation.
 35. A method according to claim 24,wherein irradiating the sample comprises irradiating a spot on thesample, and while receiving the radiation so as to generate thelow-range signal, cutting off a portion of the radiation so as to reducea dimension of the spot.
 36. A method according to claim 35, wherein theradiation comprises X-ray radiation, and wherein the range of anglescomprises angles in a vicinity of a critical angle for total externalreflection of the radiation from the surface, and wherein cutting offthe portion of the radiation comprises positioning a knife edge adjacentto the surface so as to reduce the dimension of the spot to no more than1 mm.
 37. A method for sensing radiation, comprising: receiving X-rayphotons emitted over a range of angles at an array of detector elements,so as to generate a signal indicative of respective charges accumulatedby the detector elements due to the photons that are incident on theelements; and determining, responsive to the signal, whether a high fluxof the photons or a low flux of the photons was incident on each of theelements; estimating the number of photons incident on each of theelements on which the high flux was incident by dividing the chargesaccumulated by the elements by a high-flux average charge; andestimating the number of photons incident on each of the elements onwhich the low flux was incident by dividing the charges accumulated bythe elements by a low-flux average charge, substantially different fromthe high-flux average charge.
 38. A method according to claim 37,wherein determining whether the high flux or the low flux was incidentcomprises determining that the high flux was incident on one of theelements when the charge accumulated by the element, not including abackground charge, is at least three times the high-flux average charge.39. A method according to claim 37, wherein determining whether the highflux or the low flux was incident comprises determining that the lowflux was incident on one of the elements when no more than a single oneof the photons was incident on the element over a period during whichthe charges were accumulated.
 40. A method according to claim 37,wherein estimating the number of photons incident on each of theelements on which the low flux was incident comprises dividing thecharges accumulated by a mutually-adjacent pair of the elements by thelow-flux average charge, so as to determine whether one of the photonswas incident on one of the elements in the pair.
 41. A method accordingto claim 37, wherein receiving the X-ray photons comprises the X-rayphotons reflected by a sample over the range of angles, characterized bya variation of flux of the reflected photons as a function of angle,such that the high flux is incident on the elements in a low-angleportion of the range, and the low flux is incident on the elements in ahigh-angle portion of the range.
 42. A method for detecting radiation,comprising: enclosing an array of detector elements in an enclosure, thearray including a first element and a last element defining a length ofthe array therebetween, the enclosure having a window at a front sidethereof, which is adapted to allow radiation to pass therethrough, andwhich is positioned at a distance from the array that is at least equalto the length of the array; evacuating the enclosure containing thearray; and receiving the radiation at the array and generating a signalresponsive thereto.
 43. A method according to claim 42, wherein thedistance from the array to the window is at least twice the length ofthe array.
 44. A method according to claim 42, wherein receiving theradiation comprises receiving the radiation emitted from a sampleoutside the enclosure.
 45. A method according to claim 44, wherein theradiation comprises X-rays reflected from the sample over a range ofangles, such that the first element receives the radiation reflectedfrom the sample in a lower portion of the range of angles and the lastelement receives the radiation reflected from the sample in a higherportion of the range of angles, and wherein generating the signalcomprises transferring charges generated at the detector elementsresponsive to the radiation to an output from the detector elements insequence along the array beginning with the last element.
 46. A methodfor reflectometry, comprising: irradiating a sample comprising one ormore thin film layers with radiation over a range of angles relative toa surface of the sample; receiving the radiation reflected from thesample over the range of angles and generating a signal responsive tothe reflected radiation, the signal having an oscillatory pattern as afunction of the angle, the pattern comprising an initial shoulderoccurring near a critical angle for total external reflection of theradiation from an outer one of the thin film layers at the surface ofthe sample; and estimating a density of the outer thin film layerresponsive to the shoulder, irrespective of any other properties of theone or more thin film layers.
 47. A method according to claim 46 andcomprising determining one or more of the other properties using theestimated density.
 48. A method according to claim 47, whereindetermining the one or more of the other properties comprises estimatinga thickness of at least one of the layers.
 49. A method according toclaim 47, wherein determining the one or more of the other propertiescomprises estimating a surface roughness of at least one of the layers.